There are many known types of optical recording elements. One of the currently popular elements is the compact disk (CD). Digital information is stored in the form of high optical density marks or pits on an otherwise reflective background. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the optical density is higher than in those areas not having the deformations.
It is desirable to produce recordable optical recording elements which, when recorded, produce a record that mimics the conventional CD on read out. Recording and read out is at about 780 nm. In this manner, information can be added to the CD and the CD can be used on a conventional CD player.
One element of this type is the so called "Photo CD". In the system in which this element is used, conventional photographic film is first processed in a conventional manner. Then, the images from the film are digitized and the digital information is recorded in a CD readable form on the optical recording element (Photo CD). Images can then be played back on a CD-type player into a conventional television. Since the element has a capacity for a number of digitized images that is greater than the number of images on a typical roll of consumer film, it is anticipated that the user will want to add images to a preexisting CD.
These recordable optical recording elements consist of a polycarbonate substrate containing a continuous tracking groove. An organic dye layer (recording layer) is solvent coated onto the grooved substrate. Fabrication is completed by coating a reflector layer, a protective layer, and a label in a fashion similar to a conventional CD digital audio disc or CD-ROM disc. In the recordable element the digital information is written into the dye layer with the focused beam of a diode laser operating in the near infrared region of the spectrum.
Commercially useful recordable optical recording elements have stringent requirements. The recording layer must have the required reflectivity and must also be able to couple with incident laser irradiation to provide features having adequate optical contrast. The layer must also have good stability towards light, heat and humidity for acceptable shelf life (incubation stability). Since the Photo CD is a consumer product, it must be capable of withstanding extreme environments. Between the time the original images are recorded on the Photo CD element and the time subsequent images are recorded, the element might be placed in strong sunlight.
In addition, the optical properties of the recording layer (its' refractive index) must be finely tuned especially as regards `k` (the imaginary component of the index of refraction) which controls the absorption of light at the writing laser wavelength. K must be finite but low to insure a balance between coupling and reflectivity.
Experimentally, it is difficult to find a single dye with the appropriate value of k. It is common in the art to combine symmetrical cyanine dyes (usually at least two dyes, one with a high k and one with a low k) to meet the k requirements. See U.S. Pat. No. 5,391,413. Indodicarbocyanine dyes have been used frequently. However, these dyes often have less than the desired light stability and will in fact fade to an unusable state after only a few days of intense sunlight exposure.
Optical recording layers containing metallized formazan dyes are disclosed in U.S. Pat. No. 5,294,471. These recording layers do not have sufficient capability to couple with incident laser irradiation to form the necessary contrast and sensitivity.
U.S. Pat. No. 5,547,728 discloses optical recording elements having recording layers comprising a mixture of metallized formazan dye and a cyanine dye. Such elements are useful but need improvements in recording sensitivity, incubation stability and light stability. Most of the expressly disclosed metallized formazans therein have a strong electron withdrawing group (typically NO.sub.2). This results in a relatively large absorption at 780 nm. As a result, those formazans have, as part of their complex refractive index, high k values (about 0.15). This relatively large k value has a negative impact on reflectivity when used with other recording layer dyes also having high k values.
Thus, there is a continuing need for optical recording materials that have the necessary optical characteristics such that they are CD compatible, can couple with incident laser irradiation to form features with sufficient contrast and yet are light and dark (incubation) stable.